A direct drive display device usually comprises a layer of display cells sandwiched between two electrode layers. On the viewing side of the display device, a transparent conductive coating on a plastic film is typically used as the front electrode. The non-viewing side comprises a backplane which has patterned segment electrodes and electric circuits. In the backplane structure, there are electric wires which electrically connect the segment electrodes (through conductive via holes) to connecting terminals which are then linked to a driving mechanism. The driving mechanism drives the display device.
There are several methods for fabricating the backplane. However each has its short-comings.
One of the methods involves the use of two copper foils laminated on each side of a polyimide substrate layer. Segment electrodes are formed from one of the copper foils by, for example, photolithography while blind via holes are formed through the other copper foil and the polyimide layer. Blind via holes (which are made conductive by metallization) are positioned corresponding to the segment electrodes on the opposite side. The electric wires (or electric lead lines) and connecting terminals for inter-connection are formed from the copper foil having the via holes. The segment electrodes are connected electrically to connecting terminals by the wires through the conductive via holes. The connecting terminals may be a Goldfinger type with a high density pitch. A chip-on-film (COF) may be bonded directly on the connecting terminals, utilizing anisotropic conductive film (ACF) bonding. The COF may also be built in the flexible backplane. This method simplifies the inter-connection between the backplane to other electronic components. However, the cost for such a flexible backplane is high due to the use of the polyimide substrate and the complexity of the method.
Printed flexible circuit is another method which utilizes a heat stabilized PET film as a substrate film on which via holes are drilled in a pre-defined area. Conductive paste is printed on one side of the PET substrate to form segment electrodes. Electric wires (or electric lead lines) are printed on the other side of the PET substrate and are electrically connected to the segment electrodes on the other side of the PET substrate, through the conductive via holes. Due to technical limitation of the conductive paste printing, a high density connecting terminal is not suitable to be printed on the PET substrate. As a result, an extra flexible inter-connector having high density pitch connecting terminals is needed which may be tape bonded electrically to the conductive paste printed wires on the PET substrate. The bonding may also need additional mechanical support. The via holes are usually visible from the viewing side of the display device when this type of backplane is used.
A further alternative method involves the use of a heat stabilized PET and a copper foil which is laminated on one side of the PET substrate. The segment electrodes are formed from the copper foil by photolithography. Blind via holes through the PET substrate are formed on the other side of the PET substrate, positioned corresponding to the segment electrodes on the opposite side. Conductive paste is used to form electric wirings on the PET surface, i.e., the opposite side of the copper foil. The electric wires on the PET surface are electrically connected to the segment electrodes formed from the copper foil through the conductive via holes. An extra flexible inter-connector is also needed in this method.